1. Field of the Invention
The present invention relates to a branching filter in which surface acoustic wave filters are arranged on a multilayer ceramic substrate, which defines a package, and to a communication apparatus including such a branching filter.
2. Description of the Related Art
A branching filter is an electrical component that is connected between an antenna and a transmitting power amplifier in a portable telephone or other suitable apparatus. Since the branching filter can be increasingly miniaturized as the passband thereof becomes higher, a branching filter using a surface acoustic wave (SAW) filter is known. The branching filter requires a transmitting-side spurious characteristic, i.e., a capability of removing second and third harmonic signals, which are unwanted signals generated by a power amplifier, and the demand for this requirement is increasingly stronger.
Under this circumstance, in order to improve the spurious characteristic, some inventions have been proposed in which a filter that is constituted by LC elements is connected to a transmitting SAW filter for use in a branching filter. Examples include a duplexer disclosed in Japanese Unexamined Patent Application Publication No. H6-90128 (patent document 1) and a branching filter disclosed in Japanese Unexamined Patent Application Publication No. H9-98046 (patent document 2).
Meanwhile, an approach that is commonly used for realizing a high-frequency analog circuit is discussed in xe2x80x9cTransistor Gijutsuxe2x80x9d (Transistor Technology), June 1987, (CQ Publishing Co. Ltd.; non-patent document 1). In this approach, a ground layer that is called a xe2x80x9cground planexe2x80x9d having a large area and stable potential is provided in a circuit and the grounds of circuit elements are all connected to the ground plane.
This approach holds true for the case of branching filters and is commonly applied to branching filters. Thus, the grounds of SAW filters, a matching filter, and a filter constituted by LC elements are connected to a ground layer provided within a multilayer ceramic substrate.
Patent Document 1: Japanese Unexamined Patent Application Publication No. H6-90128 (Data of Publication; Mar. 29, 1994)
Patent Document 2: Japanese Unexamined Patent Application Publication No. H9-98046 (Data of Publication; Apr. 8, 1997)
Non-Patent Document 1: Transistor Gijutsu, Jun. 1987 issue
Conventionally, for an improvement in transmitting a side spurious characteristic, optimum design conditions are determined by adjusting the characteristics of SAW filters, a matching element, and a filter constituted by LC elements.
When a further improvement is necessary, however, in reality, an approach by which characteristics in the passband of a transmitting SAW filter or a receiving SAW filter are sacrificed must be taken, which leads to a problem in that an improvement in the transmitting-side spurious characteristic is limited.
For improving the spurious characteristic, the following two possible approaches have been available. The first approach is to improve the spurious characteristic by increasing a capacitance ratio, i.e., the ratio of the motional capacitance to the shunt capacitance of the IDT in the transmitting SAW filter. With this approach, however, naturally, the passband characteristic of the transmitting SAW filter is also affected. To achieve impedance matching in the passband, it is also necessary to ensure that the capacitance ratio is within a certain range, and to increase the capacitance ratio, however, causes the passband to become capacitive, thus adversely affecting the VSWR loss. The second approach is to increase the attenuation in second and third harmonics by increasing the shunt capacitance of a low pass filter. With this approach, however, the gain of the low pass filter is deteriorated in the passband of a transmitting SAW filter, thereby resulting in increased loss in the transmitting SAW filter.
In order to overcome the problems described above, preferred embodiments of the present invention provide a branching filter that significantly improves a transmitting-side spurious characteristic and a communication apparatus including such a novel branching filter.
According to a first preferred embodiment of the present invention, a branching filter includes a receiving SAW filter, a transmitting SAW filter, and at least one matching element connected to the receiving SAW filter. The branching filter further includes a filter having at least one of an inductance element and a capacitance element. The filter is also connected to the transmitting SAW filter. The ground of the filter is provided separately from the grounds of the other circuit elements.
With this arrangement, since the branching filter includes the receiving SAW filter, the transmitting SAW filter, and the at least one matching element connected to the receiving SAW filter, the branching filter according to the first preferred embodiment of the present invention can have a branching-filter function. Additionally, since the ground of the filter having at least one of an inductance element and a capacitance element is provided separately from the grounds of the other circuit elements, this branching filter greatly improves the transmitting-side spurious characteristic without deteriorating the in-band characteristics of the transmitting SAW filter or the receiving SAW filter.
In addition, since the branching filter includes the filter connected to the transmitting SAW filter, this arrangement significantly improves the transmitting-side spurious characteristic. Furthermore, completely separating the ground of the filter from the grounds of the other circuit elements and providing an external terminal that is used only for the ground of the filter further improves the transmitting-side spurious characteristic.
As a result, with this arrangement, it is possible to provide a branching filter having an improved transmitting-side spurious characteristic without affecting the receiving- and transmitting-side band characteristics.
Preferably, the filter having at least one of an inductance element and a capacitance element is a low pass filter having a xcfx80-type circuit. In the xcfx80-type circuit, capacitance elements are connected to corresponding opposite ends of inductance elements, the inductance elements are connected in series with each other, and the capacitance elements are each connected to ground, and another capacitance element is connected in parallel with the inductance elements.
Alternatively, the filter having at least one of an inductance element and a capacitance element may be a low pass filter including two inductance elements that are connected in series with each other, a series circuit in which a capacitance element and an inductance element are connected in series with each other, and one end of the series circuit is connected between the two inductance elements.
When the low pass filter having one of the above-described configurations is used, the low pass filter can be configured to have a ripple in the filter characteristic, and matching the ripple to the passband of the transmitting SAW filter so as to prevent deterioration in the bandpass characteristic of the transmitting SAW filter.
Preferably, the receiving SAW filter and the transmitting SAW filter are provided on a ceramic substrate. With this arrangement, it is possible to provide a branching filter that is superior in robustness and that is very easy to handle.
Preferably, the ceramic substrate has a multilayer structure. With this arrangement, the matching element and the filter, which may be the low pass filter, can be provided within the multilayer substrate, thereby allowing for miniaturization of the branching filter.
Preferably, the filter, which may be the low pass filter, is provided within the multilayer ceramic substrate. This arrangement can make it easier to manufacture the branching filter than in the case in which the filter is externally provided. In addition, the transmitting SAW filter and the filter, which may be the low pass filter, can be arranged in close proximity to each other, thereby contributing to an improvement in the transmitting-side spurious characteristic.
Preferably, the ground of the receiving SAW filter and the ground of the transmitting SAW filter are separated from each other.
Another preferred embodiment of the present invention provides a communication apparatus including the branching filter described above. Since the communication apparatus according to this preferred embodiment of the present invention has a branching filter with an improved transmitting-side spurious characteristic, the communication characteristic is greatly improved.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention.